The Book of Prayer - Travellers
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 .Jabir reported: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)
From Sahih Muslim, whose reports the scholars accept as authentic.