Yes, it is a condition that the i'tikaf (spiritual retreat) is in a mosque of a congregation or a main mosque for men, or in the house mosque for women; as He, Glory be to Him, said: (And do not approach them while you are in i'tikaf in the mosques) Al-Baqarah: 187. He described them as being in i'tikaf in the mosques even though they did not engage in sexual relations in the mosques; to prohibit sexual relations there, which indicates that the place of i'tikaf is the mosque. And Aisha, may Allah be pleased with her, said: "The Sunnah for the one in i'tikaf is that he does not visit the sick, nor attend a funeral, nor touch a woman, nor engage with her, nor leave for a need except for what is absolutely necessary, and there is no i'tikaf except with fasting, and there is no i'tikaf except in a main mosque." This is found in Sunan Abu Dawood 2: 333, Sunan Al-Bayhaqi Al-Kabir 4: 321, and Musannaf Abdul Razzaq 3: 168. This applies equally to obligatory and voluntary i'tikaf; because the text is general. See: Bada'i Al-Sana'i 2: 113.