Mishkat al-Masabih 4 Prayer كتاب الصلاة 379

وَعَنْ جَابِرٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ فِي صَلَاتِهِ بَعْدَ التَّشَهُّدِ: «أَحْسَنُ الْكَلَامِ كَلَامُ اللَّهِ وَأَحْسَنُ الْهَدْيِ هدي مُحَمَّد» . رَوَاهُ النَّسَائِيّ

Jabir said that God’s Messenger used to say in his prayer after the tashahhud , “The best speech is God’s speech, and the best guidance is Muhammad’s guidance.” Nasa’i transmitted it.