There should be a gap between obligatory and supererogatory prayers

It is narrated from a companion of the Messenger of Allah that the Messenger of Allah (peace and blessings be upon him) led the Asr prayer. (After the salam) a man immediately started to pray. Sayyiduna Umar (may Allah be pleased with him) saw him and said: "Sit down, the People of the Book were destroyed because there was no interval in their prayers." The Messenger of Allah (peace and blessings be upon him) said: "Ibn Khattab has done well."

Abdullah bin Rabah narrates from a Companion that the Messenger of Allah (peace and blessings be upon him) led the Asr prayer. A man immediately stood up to offer more prayer. Sayyiduna Umar (may Allah be pleased with him) saw him and, grabbing his sheet or garment, said: "Sit down. The People of the Book were destroyed because there was no interval in their prayers." The Messenger of Allah (peace and blessings be upon him) said: "Ibn al-Khattab did well." And in another narration: "(Ibn al-Khattab) spoke the truth."