Ibrahim reports that Sayyiduna Hudhayfah (may Allah be pleased with him) said to Sayyiduna Ibn Mas'ud (may Allah be pleased with him): Are you not amazed at people who are between your house and the house of Abu Musa, claiming that they are in i'tikaf (seclusion)? Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "Perhaps they are right and you are mistaken, or they have remembered and you have forgotten." Then he said: "As for me, I know that i'tikaf is only in a mosque where the congregational prayer is (regularly) held."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9476