الذكر لعلاج القلق والهم
Remembrance of Allah (dhikr) described in the Qur'an itself as the source of the heart's rest — treated by Ibn al-Qayyim as medicine for the heart, alongside physical remedies for the body.
أَلَا بِذِكْرِ اللَّهِ تَطْمَئِنُّ الْقُلُوبُ
“Verily, in the remembrance of Allah do hearts find rest.” (meaning, not a literal translation)
Compare Sahih al-Bukhari 6407: 'The example of the one who remembers his Lord and the one who does not is that of the living and the dead.'
In Al-Wabil al-Sayyib, Ibn al-Qayyim writes at length on dhikr's effect specifically on grief, loneliness, and anxiety — treating it as a parallel discipline to his physical-medicine writing, aimed at the heart rather than the body.