'Aayeshah Radiyallahu 'Anha reports that whenever Rasulullah Sallallahu 'Alayhi Wasallam could not perform the tahajjud because of some reason. He performed twelve rak'ahs in the day (at the time of chaasht-before midday). (JA)
English reference (JA): Book 39, Hadith 254 (JA)
Arabic reference (JA): Book 40, Hadith 267 (JA)