'Abdurrahmaan bin Abi Layla RA. (a Taabi'ee) reports:
"None had given me the news that they had observed Rasulullah Sallallahu 'Alayhi Wasallam perform the salaatut duhah besides Ummi Haani. She said: Rasulullah Sallallahu 'Alayhi Wasallam came to her house, the day Makkah was conquered. He performed ghusl (bath), then performed eight rak'ahs. I did not observe Rasulullah Sallallahu 'Alayhi Wasallam perform any salaah shorter than these, despite him performing every ruku' and sajdah properly"'. (He did not perform the ruku' and saidah haphazardly because the salaah was short).
English reference: Book 40, Hadith 274
Arabic reference: Book 41, Hadith 290