What is the authenticity of the hadith prohibiting cutting nails and hairs for those who want to sactifice on eid al-adha?

What is the authenticity and interpretation of the hadith prohibiting cutting nails and hairs for those who want to sacrifice an udhiyyah on eid al-adha? AFAIK this hadith is compiled in Musnad Ahmad and one of the sunan (maybe ibn Majah or an-Nasa'i?) The hadith basically say that a person (man?) who is intending to do the sacrifice (udhiyyah) should stop cutting his nails and hair (until this is done). I'd like to understand the interpretation from these perspectives: - if the hadith is authentic how does it go along with the recommendation of being clean (tahara) etc. - when should one start (leaving hair and nails grow), once the intention or decision is made or at the moment one has bought a sheep or any other valid udhiyyah (is it a niyyah or a qasd matter?) - When should one cut his hair and nails again, right after the udhiyyah is slaughtered or later or sooner (after Eid prayer, day of Eid)? - how much should one stick on this and are there conditions allowing not to do so, lets say I just wanted to cut my nails, but if i left them 10 more days they would be too long ...