I am planning to travel to Istanbul,and I've seen that the city host the grave of Abu Ayyub al-Ansari, a companion of the Holy Prophet (s) and shia of Imam Ali (as) if I'm not mistaken.
Is there any narrations or ziyarat specific to Abu Ayyub al-Ansari that one could recite in front of his grave ? Since he is a prominent companion, can I do tawassul in his haram?

We don’t have a specific Ziyarah for him. After the Prophet (s) his stance was good towards the Ahlulbayt (a). He fought alongside Imam Ali (a) and pledged him allegiance. Some criticize him for fighting under Mu’awiyah in Turkey, but we have a hadith from al-Fadhl ibn Shadhan that excuses him and says: even though he made a mistake by fighting under Mu’awiyah (and it seems that he didn’t seek permission from Imam Hussain (a) to do that), he had good intentions and so Allah can forgive him. You can recite Sura Fatiha for his soul, pray 2 rak’as and gift them to his soul, and recite Qur’an for him.