Tawus (may Allah have mercy on him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) passed by Abu Isra'il (may Allah be pleased with him), who was standing in the sun, and said: "What is the matter with him?" They said that he had vowed that he would not sit, nor take shade, nor speak to anyone, and that he would fast. So he (peace and blessings be upon him) said: "Let him take shade, sit down, speak to people and complete his fast," and he (peace and blessings be upon him) did not order him to make expiation.
This is a good mursal report.
It has also been narrated from Muhammad bin Kurayb, from his father, from Ibn Abbas (may Allah be pleased with them both), with the order to make expiation in it; Muhammad bin Kurayb is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20093