Sahih Muslim 756
Sahih Muslim · Book 6: The Book of Prayer - Travellers · The best prayer is that in which one stands for a long time (tuwlul-qunut)
Jabir reported:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا أَبُو مُعَاوِيَةَ ، حَدَّثَنَا الأَعْمَشُ ، عَنْ أَبِي سُفْيَانَ ، عَنْ جَابِرٍ ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَىُّ الصَّلاَةِ أَفْضَلُ قَالَ " طُولُ الْقُنُوتِ " . قَالَ أَبُو بَكْرٍ حَدَّثَنَا أَبُو مُعَاوِيَةَ عَنِ الأَعْمَشِ .
The Messenger of Allah (ﷺ) was asked about the prayer which was most excellent. He said: That in which the standing is longer. (This hadith is narrated by another chain of transmitters too.)