Ruling on fasting the days of al-Tashreeq

I have the habit of fasting every Thursday and it so happened that I
fasted on Thursday 12th Dhu'l-Hijjah. On the Friday I heard that it is
not permissible to fast on the Days of al-Tashreeq and that Thursday
was the third of the days of al-Tashreeq. Is there any sin on me if I
fasted that day? Is it true that it is not permissible to fast on the
days of al-Tashreeq or should we avoid fasting only onthe first days
of Eid?.
Praise be to Allaah.
Fasting on the two days of Eid is haraam, as indicated by the hadeeth
of Abu Sa'eed al-Khudri (may Allaah be pleased with him) who said: The
Prophet (peaceand blessings of Allaah be upon him) forbade fasting on
the day of al-Fitr and al-Nahr." Narrated by al-Bukhaari,1992; Muslim,
827. The scholars are unanimously agreed that fasting on these two
days is haraam.
It is also haraam to fast on the days of al-Tashreeq which are the
three days after Eid al-Adha (the 11th, 12th and 13th of Dhu'l-Hijjah)
because the Prophet (peace and blessings of Allaah be upon him) said:
"The days of al-Tashreeq are days of eating, drinking and remembering
Allaah." Narrated by Muslim, 1141.
Abu Dawood (2418) narrated from Abu Murrah the freed slave of Umm
Haani that he entered with 'Abd-Allaahibn 'Amr upon his father 'Amr
ibn al-'Aas. He offered them food and said, "Eat." He said, "I
amfasting." 'Amr said: "Eat, for these are the days onwhich the
Messenger of Allaah (peace and blessings of Allaah be upon him) would
tell us not to fast and forbade us to fast, and these are the days of
al-Tashreeq."Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
But it is permissible to fast on the days of al-Tashreeq for a
pilgrimwho does not have a hadiy (sacrificial animal).It was narrated
that 'Aa'ishah and Ibn 'Umar (may Allaah be pleased with them) said:
No one was allowed to fast on the days of al-Tashreeq except one who
could not find a hadiy. Narrated by al-Bukhaari,1998.
Shaykh Ibn 'Uthaymeen said: It is permissible for those who are doing
Hajj qiraan or tamattu', if they cannot find a hadiy, to fast these
threedays so that the Hajj season will not end before they fast. But
apart from that it is not permissible to fast on these days, even if a
person is obliged to fast for two consecutive months; he should not
fast on the day of Eid and the three days following it, but then he
should continue his fast.
Fataawa Ramadaan, p. 727.