Allaah - the Most High - said:
"Indeed those who say: Our Lord is Allaah, and then do istiqaamah (be upright and obedient) upon them shall be no fear, nor shall they grieve. Those will be the companions of Paradise, to dwell therein forever - a reward for what they used to do." [Soorah Ahqaaf 46:13].
From Abu 'Umar Sufyan ibn 'Abdullaah radiallaahu 'anhu, who said: O Messenger of Allaah sallallaahu 'alayhi wa sallam! Tell me something about Islaam which I can ask of no one but you? So he said: "Say: My Lord is Allaah and then do istiqaamah (be upright)." Abu 'Umar then asked him: O Messenger of Allaah! What do you fear that is the most harmful to me? So he took hold of his tongue and said: "Control this." 1
Abu Bakr as-Siddeeq radiallaahu 'anhu said, whilst explaining the meaning of the aayah (verse): "thereafter do istiqaamah" said: "Not associating any partners in the worship of Allaah." And he said: "Not turning to other than Him."
Abul-'Aaliyah (d.90H) - rahimahullaah - said:
"To make the religion and one's actions pure and sincere for Allaah alone."
Qataadah (d.104H) - rahimahullaah - said:
"To stand firm upon obedience to Allaah." 2
Ibn Rajab (d.795H) - rahimahullaah - said: 3
"The foundation of istiqaamah is: firmness of the heart upon tawheed (i.e. firmness in worshipping Allaah alone, not worshipping others along with Him) - as Abu Bakr and others have explained.
Thus when the heart is firm and upright upon the ma'rifah (realisation and knowledge) of Allaah, and upon fear, veneration and awe of Him, and upon love, desire and hope in Him, calling upon Him, and repenting and turning away from everything besides Him - then the limbs will all follow in obedience to Him. This is because the heart is the king of the limbs and they are its army. So if the king is upright, then his army and subjects will be upright.
Then, after the heart, the greatest of the body-parts which takes care of one's istiqaamah is the tongue - since the tongue is the translator of the heart and is its interpreter (i.e. the tongue is like a ladle which scoops-out what is in the heart). This is why, when the Prophet sallallaahu 'alayhi wa sallam ordered Abu 'Umar with istiqaamah, he then ordered him to safeguard his tongue.
The Prophet sallallaahu 'alayhi wa sallam also said:
"The eemaan (faith) of a person will not be upon istiqaamah (firmness and obedience to Allaah), until his heart is upon istiqaamah. And his heart will not be upon istiqaamah, until his tongue is upon istiqaamah." 4
And he sallallaahu 'alayhi wa sallam said:
"When the son of Aadam wakes up in the morning, then all of the body-parts humble themselves to the tongue, saying: Fear Allaah with regards to us, for we are dependant upon you. If you have istiqaamah, then we have istiqaamah, and if you swerve, then we also swerve." 5"
1. Related by Muslim (no.31) and at-Tirmidhee (no.2535).
2. Ibn Katheer related these narrations in his Tafseer Qur'aanul-'Adheem (4/106-107).
3. Jaami'ul-'Uloom wal-Hikam (pp.312-313), abridged.
4. Related by Ahmad (3/198) with a Hasan isnaad.
5. Hasan: Related by at-Tirmidhee (no.2531), from Abu Sa'eed al-Khudree radiallaahu 'anhu. It was authenticated by al-Albaanee in Saheehul-Jaami' (no.351).