From the kind of permissible Tawassul: that a person asks someone to supplicate to Allaah, The Most High, for him, if it is hoped for that this person’s supplication will be answered.
However, it should be that the person requesting someone to supplicate Allaah for him intends to benefit himself and his brother, the one from whom he asked the supplication, so that the request is not solely for his own interest. Since if you want to benefit your friend as well as yourself, then this will end up as something good for him.
This is because when a man supplicates to Allaah for his brother in his absence, the angel says: “Aameen, and for you the same.” Similarly when someone supplicates Allaah for his brother, then he is from those who do good by virtue of this supplication, and Allaah loves those who do good.”
Translated by Dr Saleh as Saleh rahimahullaah
Source : Understanding Worship – Fiqh ul-‘Ibadah – Shaykh ibn Uthaymeen – Page 159