All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We could not find what the questioner has mentioned neither in the books of Sunnah, nor in the books of Ibn Taymiyyah . So we do not know its source. It is probable, however, that the questioner meant the narration reported by At-Tirmithi which was narrated by 'Aa'ishah as she said: ''The Prophet said: ''Eed Al-Fitr is when people break their fasting, and 'Eed Al-Adh-Haa is when people slaughter their sacrifice''. So the narration does not state "and the Hajj is when people perform Hajj."
Therefore, if this is the intended narration, it was stated by Ibn Taymiyyah when he was speaking about the sighting of the moon of Thul-Hijjah and the standing of pilgrims on the mount 'Arafah on the tenth day by mistake, is still acceptable from them (that serves instead of the 9th day of Thul-Hijjah).
Allaah knows best.