It's covered in Rule 6-8d(iii):
d. Procedure When Play Resumed
Play must be resumed from where it was discontinued, even if resumption occurs on a subsequent day. The player must, either before or when play is resumed, proceed as follows:
(i) if the player has lifted the ball, he must, provided he was entitled to lift it under Rule 6-8c, place the original ball or a substituted ball on the spot from which the original ball was lifted. Otherwise, the...