Shama-il Tirmidhi Chapter 039, Hadith Number 007 (254).

'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).

Commentary
If tahajjud was fard on Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam then it is apparent that he performed qadaa. If it was not fard then he performed qadaa because of it being meritorious and full of virtue.' In the Sahih Muslim, Sayyidina 'Umar Radiyallahu 'Anhu relates a saying of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam that 'If a person could not fulfil his wird (daily practices) and ma'mul (regular practices) at night, one should complete them anytime in the morning before noon. It will be just as if he had completed them at night'. The mashaa-ikh of suluk (spiritual guides) take from this that if one cannot get through one's daily and regular practices at night, one should complete them in the morning. Because of the time passing by, one should not ignore it, otherwise the nafs will become accustomed to it.