Sufyan bin 'Abdullah Ath-Thaqafy reported that he said,

“I said, ‘O Messenger of Allah, tell me about Islam a thing which might dispense with the necessity of my asking anybody after you.’ Prophet (Sallallahu A’laihi wa sallam) remarked, ‘Say I affirm my faith in Allah and then remain steadfast to it.’ ”

( Recorded by Imam Muslim in Kitab Al-lman (Book on Faith), no. 62/38; also recorded by Imam Ahmad in Al-Musnad, no. 2/413.)

[size=14pt]Read it’s explanation from Jami’ Al Ulum wal Hikam by Ibn Rajab al Hanbali[/size]