Hadith 2509
وَعَنِ ابْنِ عَبَّاسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «إِن عمْرَة فِي رَمَضَان تعدل حجَّة»
Ibn ‘Abbas reported God’s messenger as saying, “An 'umra in Ramadan is equal to a hajja. ” (hajj is the word for the pilgrimage and hajja is a form indicating a single observance.)