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Part of the answer to this question what can a wizard do without a spellbook got me wondering. It states that you, as a wizard, can't cast a ritual spell without your spellbook.

And I can't help question if that is true?

Because the rules doesn't actually seem to state that explicitly.

Let's look at the rules for rituals in the PHB.

PHB pg.202

The caster must also have the spell prepared or on his or hers list of spells known, unless the character's ritual feature specifies otherwise, as the wizard does.

So while those rules doesn't require a spellbook, they do single out the wizard's ritual feature.

So what does the wizard ritual feature say?

PHB pg. 114

You can cast a wizard spell as a ritual if that spell has the ritual tag and you have the spell in your spellbook. You don't need to have the spell prepared

Now while the rules mentions a spellbook, they don't actually state that you need to use it during the ritual casting. Just that the spell already needs to be inscribed in the spellbook. Like how the general rules for rituals states that a spell needs to be in the casters known list of spells.

And if you look at the wording it it is very similar to other wizard features that require you to have the spell in your spellbook but doesn't require you to access your spellbook while casting.

For example the Spell Mastery feature:

PHB. pg. 115.

Choose a 1st-level wizard spell and a 2nd-level wizard spell that are in your spellbook. You can cast those spells at their lowest level without expending a spell slot when you have them prepared.

or the Signature spell feature:

PHB: pg. 115.

Choose two 3rd-level spells in your spellbook as your signature spells.

In both examples the rules seem to treat the spellbook as a refernce to the spells known by the wizard, identically to the known spell list other caster classes use. And not as a physical thing the wizard are actually accessing at that moment to use those features.

Then compare it to the Arcane Recovery feature that explicitly states that you need to be using the spellbook for that feature to work.

PHB. pg. 115

You have learned to regain some of your magical energy by studying your spellbook.

There the rules explicitly states that the wizard must be using the actual physical spellbook for the feature to work.

So by comparing the RAW for rituals in general and the different wizard features there doesn't seem to be anything that says that a wizard needs to be using the actual physical spellbook while casting a ritual spell as a ritual.
And while it from a certain logical and thematically viewpoint it make sense for a wizard to read for the spellbook while performing a ritual, nothing in the RAW seem to support it.

So can a Wizard cast a spell as a ritual without physically accessing a spellbook?
And is there an official ruling, either way?

Technically, they do not

As per D&D 5e's paradigm of rules interpretation, the rules say what they say and no more.

While the warlock with Book of Ancient Secrets specifically requires their Book of Shadows in hand to cast rituals, and the Ritual Caster feat requires the user to have their ritual book in hand, no actual rule in the Player's Handbook defines that the wizard must have their book in hand.

Certainly, most DMs would expect the wizard to have their spellbook to cast rituals, as it makes little sense to be able to cast an unprepared spell based only on the fact that you scribed it in a book once years ago, but this is not clearly defined in the rules.

The wizard's Ritual Casting feature (p.114) defines only as follows:

You can cast a wizard spell as a ritual if that spell has the ritual tag and you have the spell in your spellbook. You don't need to have the spell prepared.

You might broadly interpret this to mean that you must "have" the spell to hand (in your spellbook), but whether this meaning is intended this is unclear.

At time of writing, nobody's specifically asked Jeremy Crawford if a wizard needs their spellbook in hand to use rituals. His answers on the topic only state that you can cast wizard rituals that are in your spellbook and that you don't need a spell prepared to cast it from your spellbook. Some have interpreted the second quote to mean that you need your spellbook to cast "from" it, but this is not clearly stated.