Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) something similar to the saying of Mujahid: When they become intermingled, they should make remembrance (dhikr) and gesture with the head. Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "If they are more than that, then they should pray standing and riding."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6022