Mishkat al-Masabih 4 Prayer كتاب الصلاة 228

وَعَنْ جَابِرٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَفْضَلُ الصَّلَاةِ طُولُ الْقُنُوتِ» . رَوَاهُ مُسلم

Jabir reported God’s Messenger as saying, “The most excellent prayer consists in long standing in prayer.” Muslim transmitted it.