(8) The Book of Virtues
كتاب الفضائل
(212) Chapter: Excellence of Standing in Prayer at Night
(212) - باب فضل قيام الليل‏:‏
Riyad as-Salihin 1176 
Jabir (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) was asked: "Which Salat is the best?" He replied, " The best Salat is that in which Qiyam (the duration of standing) is longer."

[Muslim].

وعن جابر رضي الله عنه قال‏:‏ سئل رسول الله صلى الله عليه وسلم ‏:‏ أي الصلاة أفضل‏؟‏ قال‏:‏ ‏ "‏طول القنوت‏"‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏
Reference : Riyad as-Salihin 1176
In-book reference : Book 8, Hadith 186