Asātarūpa Jātaka (No.100)
Once the Bodhisatta was King of Bārāṇasī. The Kosala king waged war on him, slew him and bore off his queen to make her his own wife. The king’s son escaped through a sewer and later came back with a large army to give battle. His mother, hearing of his doings, suggested that he should blockade the city instead.
This he did, and the blockade was so close that on the seventh day the people cut off the head of the king and brought it to the prince.
It was this prince who became Sīvalī in the present age; the blockade was the reason for his remaining seven years in his mother’s womb, and for her being seven days in bringing him forth. His mother was Suppavāsā, daughter of the Koliya king.
The story was related by the Buddha to explain to the monks the reason for Suppavāsā’s long pregnancy. J.i.407‑10. This Jātaka appears, with variations in detail, in DhA.ii.198 ff.