Sayyiduna Ibn Abbas (may Allah be pleased with them both) said concerning the saying of Allah Almighty: "That they may increase in faith along with their faith" [Surah al-Fath: 4]: "Allah, the Mighty and Majestic, sent His Prophet (peace and blessings be upon him) with the testimony 'La ilaha illallah' (there is no god but Allah). When they affirmed it, He added Hajj for them; then when they affirmed that as well, He added jihad for them; then He perfected their religion for them," and so He said: "This day I have perfected for you your religion and completed My favour upon you" [Surah al-Ma'idah: 3]. Ibn Abbas (may Allah be pleased with them both) said: "So the firmest, truest and most complete faith of the people of the heavens and the people of the earth is the testimony 'La ilaha illallah'."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12972