It is recommended that his intention be made after the prayer while he is sitting without a significant interruption before he stands up, mounts, or walks. However, if he enters the state of ihram after he has walked or mounted, it is permissible; as narrated by Ibn Umar (may Allah be pleased with him), he said: "The Prophet (peace be upon him) declared the pilgrimage when his mount was standing upright." [In Sahih al-Bukhari 2: 562]. See: Al-Labab and Al-Maslak, pp. 102-103.