The ‘ulama give two meanings of the phrase ‘long part of the night’. One is that a long portion was recited whilst standing and a long portion whilst sitting. The second is that this is the description of different nights. On some nights he recited lengthy sections whilst standing and on some nights he recited lengthy sections whilst sitting. The second explanation is more appropriate, which means it was the practice of Sayyidina Rasulullah Sallallahu ‘Alayhi Wasallam always to recite lengthy sections in the rak’ahs of tahajjud. If he sometimes felt weak he sat and performed the tahajjud, otherwise he stood and performed it.
The last section of this hadith seems to contradict the previous one. The ‘ulama have mentioned that both incidents are on separate occasions. The stipulation of nawaafil is on strength, willingness and happiness. Whatever strength and temperament was experienced at the time, salaah was performed accordingly. Besides, many acts of Sayyidina Rasulullah Sallallahu ‘Alayhi Wasallam were meant to educate. He performed many deeds to let the people know that certain deeds could be performed in a particular manner. This is known as bayan jawaaz (sign of permissibility). Therefore Sayyidina Rasulullah Sallallahu ‘Alayhi Wasallam received the full reward for performing salaah in a sitting posture. Others will receive full reward for performing salaah in a standing posture and only half for sitting and performing salaah. The reason being that by Sayyidina Rasulullah Sallallahu ‘Alayhi Wasallam sitting and performing salaah, he was fulfilling an object of the nubuwwah (prophethood). The aim and mission of a nabi is to give a practical demonstration on how deeds are to be performed.