Al-Ḥāfiẓ Ibn Rajab (d. 795 AH) said: “The truth regarding the difference [between Islām and īmān] is that īmān is to believe with the heart in the truthfulness of this dīn, to be satisfied with it, and to have knowledge concerning it. While Islām refers to the complete submission, humility, and humble obedience of the servant [of Allāh]. This is enacted through action and is referred to as dīn by Allāh in His Book1. In the ḥadith of Jibrīl the Prophet (ﷺ) referred to Islām, īmān, and īḥsān [God- consciousness] as dīn2.
This is evidence that proves that if one of these two terms [Islām or īmān] is ever cited in isolation from the other, the single term will take on the meaning of both words. Differentiation between them is only applicable if both terms are mentioned together in the same context. In such a case, īmān will refer to bearing witness as to the truthfulness of the dīn with the heart, and Islām will apply to the physical actions [of the dīn].
In the Musnad of Imām Aḥmad on the authority of Anas (may Allāh be pleased with him), the Prophet (ﷺ) said: ‘Islām is seen, and īmān is in the heart.’ This is because actions are publicised and easily seen, while bearing witness to the truthfulness of the dīn itself is in the heart and, thus, is unseen.3”
 ‘Indeed, the religion in the sight of Allāh is Islām.’ [Āli ʿImrān, 3:19]
 That was Jibrīl. He has come to teach you your religion’ [Ṣaḥīḥ Muslim: 8, Ṣaḥīḥ Bukhārī: 50]
 Musnad 3:134-135
Source: Jāmiʿ al-ʿUlūm wa-al-Ḥikam: 111
Translated by: Riyāḍ al-Kanadī