Use of instrumental with был/быть, e.g. "Ты был забавным."

Question

Today the following sentence came up in a chat with a Russian friend:

Ты был забавным.

Here the adjective is in dative instrumental case, but I would expect it to be in nominative together with the verb был/быть.

When I asked my friend why she didn't write Ты был забавный, she explained that this was because it was in past tense. But I cannot find this rule anywhere in Russian grammar books.

Can anybody explain the use of dative instumental in this sentence?

Edit: I mistakenly wrote dative case above. I meant instrumental case.

Answer 1

The noun and adjective functioning as the nominal part of a compound predicate can be in the nominative or in the instrumental case. Typically, the nominative here [where the linking verb быть is present] corresponds to a permanent quality of the subject while the instrumental case better combines with a temporary quality.

In much more detail (Розенталь Д.Э., Джанджакова Е.В., Кабанова Н.П.CПРАВОЧНИК ПО ПРАВОПИСАНИЮ, ПРОИЗНОШЕНИЮ, ЛИТЕРАТУРНОМУ РЕДАКТИРОВАНИЮ, M.: ЧеРо, 1999): http://www.evartist.narod.ru/text1/62.htm#%D0%B7_02

(see §179. Формы сказуемого)

Answer 2

What can be added to the answers in the past topics referenced by Sergey Slepov in the comments, is that at least in your example, usage of adjective in nominal or instrumental affects semantics of the phrase.

Ты был забавный ^Nom. would relegate your being забавный to a distant past, when there was another you so to speak, for example in the childhood, youth etc, that is once you were so. That's probably agrees with the fact of it indicating permanent (or stable within certain time limits) quality as mentioned elsewhere.

Ты был забавным ^Instr. can be applied to any time span.