Abu Dawud Book 1, Hadith Number 158.
Chapter : Time limit for wiping over the shoes.
Narated By Ubayy ibn Umarah : I asked: Messenger of Allah (PBUH) may I wipe over the socks? He replied: Yes. He asked: For one day? He replied: For one day. He again asked: And for two days? He replied: For two days too. He again asked: And for three days? He replied: Yes, as long as you wish.
Abu Dawud said: Another version says: He asked him about the period until he reached the period of seven days. The Messenger of Allah (PBUH) replied: Yes, as long as you wish (i.e. there is no time limit.
Abu Dawud said: There is a variance in the chain of narrators of this tradition. The chain is not strong.
Another chain from Yahya b. Ayyub is also disputed.