Reference : Hisn al-Muslim 16

بِسْمِ اللهِ ، تَوَكَّلْـتُ عَلى اللهِ
وَلا حَوْلَ وَلا قُـوَّةَ إِلاّ بِالله
Bismillāhi, tawakkaltu `alallāhi, 
wa lā ḥawla wa lā quwwata illā billāh.

In the Name of Allah, I have placed my trust in Allah, 
there is no might and no power except by Allah.

Reference:
Abu Dawud 4/325, At-Tirmidhi 5/490. See also Al-Albani, Sahih At-Tirmidhi 
3/151

Chapter: Remembrance when leaving the home